1. Field of the Invention
The present invention relates to electrolytic oxidation of a blix solution. More particularly, the present invention relates to a regeneration of a used blix solution and the electrolytic cell materials and procedures for use in the electrolytic oxidation of a photographic blix solution.
2. Description of the Prior Art
A silver halide color photographic material is processed in the two steps of color-development and de-silvering. On color-development of the color photographic material, the exposed silver halide is reduced to form silver, and at the same time, the oxidized aromatic primary amino developing agent reacts with a coupler to form a color image. The silver produced by the development is, at the de-silvering step, oxidized by a bleaching agent (oxidizing agent) into a silver halide which is then dissolved in and removed by a silver halide solvent.
In this way, the de-silvering stage involves the two steps of bleaching and fixing. In practical photographic processing, a blix stage in which both the bleaching step and the fixing step are carried out simultaneously is employed, as well as a method in which the bleaching step and the fixing step are carried out separately. The former method is described in, for example, U.S. Pat. No. 3,582,322.
The iron (III) chelate compound contained in the blix solution oxidizes the silver developed during the processing, and at the same time, the iron (III) chelate compound is reduced a di-valent iron salt. On the other hand, the silver ions produced by the oxidation are dissolved in a fixing agent. Therefore, with continuing processing, the iron (II) salt and silver complex salt gradually accumulate in the blix solution, and as a result of the accumulation, the activity of the blix solution is reduced, with fatigue thus occurring. This problem can be solved by adding a suitable amount of a supplemental solution to the blix solution during the development processing and by overflowing the used solution from the tank. This method, however, is not desirable from the standpoint of environmental pollution, and also from an economical standpoint. Thus it is quite desired and advantageous that the activity of the solution be restored so as to enable the solution to be reused.
In general, a method of recovering silver from the fixing solution can be utilized in recovering silver from the blix solution.
The following silver recovery methods are well known in the field of photography.
1. A method in which a compound capable of forming an insoluble silver salt is added to the solution (silver precipitation method)
2. A method in which a metal having a higher ionization tendency than silver is contacted with the solution (metal substitution method)
3. A method in which silver is deposited on the cathode in an electrolytic bath (electrolytic method)
4. A method in which an ion exchange resin is used (ion exchange method)
The details of these methods are described in M. L. Schreibe, "Present Status of Silver Recovery in Motion-Picture Laboratories", J. SMPTE, Vol. 74, pp. 505 to 514 (1965).
On the other hand, the following methods of oxidizing iron (II) ions produced in the used solution have been proposed.
1. A method in which an oxidizing agent is added, as described in, for example, U.S. Pat. Nos. 3,615,507 and 3,767,401, and West German Pat. (OLS) No. 2,149,314.
2. A method in which oxygen (air) is contacted with the solution, as described in, for example, U.S. Pat. Nos. 3,634,088, 3,700,450, and 3,813,246.
3. An electrolytic oxidation method as described in, for example, Japanese Pat. (OPI) No. 18191/1973.
In oxidation methods (1) and (2) above, an oxidizing agent is added, and sulfite ions and thiosulfate ions contained in the solution are oxidized. Thus unwanted materials accumulate in the regenerated solution. Therefore, the removal of the unwanted materials accumulated and replenishment of the deficiency in the desired components is required in these methods. On the other hand, in the electrolytic oxidation method (3), since the oxidation proceeds without any chemical reactions occurring, iron (II) ions alone are efficiently converted into iron (III) ions. Thus the electrolytic oxidation method is a preferred method.
However, if the silver recovery is carried out simultaneously with the oxidation of the iron (II) in accordance with the electrolytic oxidation method, the reduction of the iron (III) ions takes place simultaneously with the reduction of the silver complex ions in the reduction process, i.e., on the cathode, and thus a large oxidation current is needed at the anode. The reason for this is that if the oxidation is insufficiently carried out, the blix solution reproduced does not have a sufficient bleaching action. Furthermore, at the anode, the sulfite ions and thiosulfate ions are oxidized at the same time. Therefore, it is quite insufficient from an industrial standpoint for the oxidation of the iron (II) ions and the silver recovery to be simultaneously carried out using an electrolytic method.
In particular, if the thiosulfite and sulfate ions are decomposed, the blix solution must be supplemented with the corresponding amount of these ions decomposed, and thus this method suffers from the serious problem that the advantage of the regeneration operation is lost from an economical standpoint.
Furthermore, the accumulation of the sulfate ions and the like produced by decomposition causes the clearing time to increase, and the sulfur ions produced by reduction cause silver sulfide to be formed.